1. Field of the Invention
The present invention relates to a surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave filters (SAW filters) used for RF's (radio frequencies) are generally required to work on a wide frequency band and to have a low loss. Accordingly, lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), both of which have high electromechanical coupling coefficients, is used for the piezoelectric substrate. Since such a piezoelectric substrate has a pyroelectric effect, if pyroelectric charges generated by temperature changes are built up on the excitation electrodes, electrostatic discharge occurs between the electrodes and thus breaks them. In order to prevent such pyroelectric breakage of the electrodes, a manufacturing process uses a method in which electrodes are electrically connected to each other with a conductor so that they have the same potential. The conductor is to be removed afterwards. The manufacturing process is thus complicated. In addition, since the electrodes are electrically separated from each other in a final step, the pyroelectric breakage of the electrodes cannot be prevented afterwards. Furthermore, pyroelectric charges may be generated by heat in the step of mounting a surface acoustic wave filter on the circuit board of, for example, a portable device, and consequently may disadvantageously cause other electronic components to break.
Patent Document Japanese Unexamined Patent Application Publication No. 11-92147 disclosed that the buildup of surface charges on lithium tantalate or lithium niobate is reduced by controlling the crystal structure.
Use of piezoelectric substrates made of those materials that do not build up pyroelectric charges can prevent the pyroelectric breakage of electrodes, and makes the pyroelectric effect insignificant. It is therefore believed that special steps can be eliminated which are performed as measures against pyroelectric breakage, for example, the step of electrically connecting the electrodes.
However, even the piezoelectric substrates in which the pyroelectric effect should be insignificant can restore the pyroelectric effect in practice, and consequently pyroelectric charges cause problems in some cases.